The act of sexual intercourse with a pressure ulcer, e.g. of a quadriplegic or a debilitated senior. Often occurring in a skilled nursing facility.

"That caregiver is giving Mrs. Jones in room 402-A a lot of special attention. I hope he's not planning on decubing her."
or
"Did you see the tight ulcer on that chick in the wheelchair? Man, I'm going decubing tonight!"